The present invention generally relates to a device for reducing undesirable moisture in enclosures containing a heat source, such as automobile lamps and other electronic enclosures.
Many items are susceptible to damage caused by excessive moisture. As used herein, the term xe2x80x9cmoisturexe2x80x9d is intended to refer to water which is diffused or condensed, whether in liquid form or vapor form, from the ambient atmosphere. For instance, electrical and electronic items may be ruined or altered due to excessive moisture. Similarly, enclosed components, e.g., those contained in a housing, that undergo thermal cycling are susceptible to moisture related problems. Examples of enclosures which are subject to undesirable moisture include, for example, automotive headlamp units, electronics contained in enclosed housings, and other systems where on/off cycling of a heat source within an enclosure results in moisture build-up during such thermal cycling. One means of dispensing moisture is to provide greater airflow across, or through, the enclosure. However, when components are located in an enclosed housing it can be very difficult to provide adequate airflow to reduce moisture. For example, attempts to increase the vent opening sizes can exacerabate problems such as contamination, etc.
Another means of managing moisture in an enclosure is to place a drying agent or desiccant within the enclosure. As the terms xe2x80x9cdesiccantsxe2x80x9d or xe2x80x9cdrying agentsxe2x80x9d are used herein, they are intended to refer to any material which absorbs water vapor from the air and is thereby able to reduce the moisture in the air in enclosed containers.
G.B. Patent No. 1,394,411, entitled xe2x80x9cImprovements In Or Relating To Vehicle Headlamps,xe2x80x9d is directed to an air-impermeable desiccant-filled tube having an opening in one end positioned in the interior of an automotive headlamp, an opening at the other end of the tube with a diffusion damper and/or a throttling device, such as a ceramic plug, and an external diffusion tube or path which allows air from the enclosure to pass while protecting the enclosure against contamination and water splashes.
G.B. Patent No. 1,472,189, entitled xe2x80x9cImprovements In Headlamps,xe2x80x9d teaches an insertable plug which is filled with a hydroscopic agent. The plug has an opening in the interior headlamp and a porous web on the other end which is exposed to the atmosphere. In use, the device is mounted in the headlamp reflector from the exterior of the enclosure.
G.B. Patent No. 1,547,863, entitled, xe2x80x9cLamp For A Motor Vehicle,xe2x80x9d discloses a moisture reducing device in a pressure equalization passage in the cap or in an element attached to the cap. The invention employs a spiral diffusion channel, hydroscopic sheet and covering cap that protects the hydroscopic sheet from dirt and dust.
U.S. Pat. No. 4,739,458, entitled xe2x80x9cVehicle Lamp Device,xe2x80x9d discloses a flat, board-like structure that contains a desiccant-filled tube that connects the interior of the lamp to the external atmosphere. Connected to the external end of the desiccant filled tube is a rubber hose that is bent at an acute angle to prevent the direct contact of water drops, dust, mud or the like. In this rubber hose is a permeable plug that prevents contamination and controls the air flow. This patent discloses the use of the heat from the lamp to control the amount of moisture in the enclosure by thermal cycling and the exchange of air through the desiccant.
U.S. Pat. No. 4,755,917, entitled xe2x80x9cHeadlight for Vehicles, In Particular Motor Vehicles,xe2x80x9d discloses the use of a desiccant to absorb moisture from the interior of a headlamp. It incorporates a long tube with desiccant chambers located in the interior of the headlamp. The tube as well as the desiccant chambers provides a torturous path for air traveling from the interior to the exterior of the headlamp. The desiccant chambers may be covered on one side with a layer that has a high permeability for water molecules in the mixture of water vapor and air.
U.S. Pat. No. 4,796,163, entitled xe2x80x9cMotor Vehicle Headlight,xe2x80x9d discloses an air exchange device that includes an inlet valve for admitting air into the interior of the headlamp when xe2x80x9cunderxe2x80x9d pressure occurs and another valve which permits air to flow out from the interior of the headlamp into the atmosphere when xe2x80x9coverxe2x80x9d pressure occurs. The unit is filled with a desiccant that is regenerated by the heat of the lamp.
U.S. Pat. No. 4,809,144, entitled xe2x80x9cVehicle Headlamp With A Venthole,xe2x80x9d is directed to a headlamp with an internal elongated tube having small openings in the front and rear ends for communications with the interior and exterior environment. The elongated tube is filled with desiccant and may further include air permeable resin films at the interior and exterior openings, with the resin film on the exterior opening having higher permeability than the interior opening film. The desiccant filled tube is located above the lamp of the headlight.
PCT Publication No. WO 97/27042, entitled xe2x80x9cCombination Desiccant And Heat Source,xe2x80x9d is directed to mixing a desiccant with a porous polymer, forming said composite into a shape which can be placed or affixed proximate to a heat source. The device incorporates a shielded vent with an air-permeable membrane.
PCT Publication No. WO 00/47932, entitled xe2x80x9cHeadlight Assembly Humidity Control System,xe2x80x9d teaches a device inserted through the wall of the housing which connects the interior of the lamp housing to the exterior atmosphere. Air is exchanged by thermal cycling of the headlamp through the external diffusion channel, desiccant, and air permeable, microporous membrane of the device. The device may be fitted with a further protective cap to shield the air-permeable, microporous hydrophobic membrane from water and other contaminants.
The teachings of the prior art devices described above typically rely on the thermal cycling of the headlamp to reduce moisture through the mechanism of air exchange and are not particularly effective at absorbing moisture from the lamp housing itself, particularly in the case of plastic housings, when moisture is diffusing through or deadsorbing from the housing.
Accordingly, there remains a need in the art for an improved moisture reduction device incorporating a desiccant located within the enclosure adjacent the heat source to absorb and expel moisture from the enclosure while also providing protection against contamination entering the enclosure.
It has been surprisingly discovered that the majority (e.g., greater than about 50%) of moisture in plastic enclosures, such as headlamps, deadsorbs from and diffuses through the plastic housing itself. While moisture does enter an enclosure through vent openings via air exchange during temperature changes, attributed to the on/off cycling of the heat source, it is small in comparison to the amount of water that deadsorbs from and/or diffuses through the plastic housing. For example, it was found that in a plastic enclosure, such as an automobile headlamp, when the heat source is energized the plastic itself deabsorbs moisture both into the enclosure and outside into the surrounding external atmosphere. Conversely, when the energy source is deenergized, the plastic reabsorbs moisture both present in the enclosure and from the external surrounding environment. The change in moisture level can be seen within minutes of the temperature change of the heat source. It was also found that the moisture permeation through the lamp housing occurs over a long time period, from hours to days to weeks, depending on the housing construction, venting device and external ambient environment.
Accordingly, there remains a need in the art for an improved moisture reduction device incorporating a desiccant located within the enclosure adjacent the heat source to absorb and expel moisture from the enclosure while also providing protection against contamination entering the enclosure.
The present invention is a device for reducing moisture in an enclosed housing containing a heat source, such as a headlamp or other enclosure containing electronics. The invention comprises a housing for holding a desiccant, an air-impermeable, water vapor-permeable layer incorporated with said housing, preferably oriented on a side adjacent the heat source, a vent hole in the housing to provide an air pathway from inside the electronic enclosure, a diffusion tube or channel oriented with the housing to provide a pathway from the desiccant to the external atmosphere outside the electronic enclosure, and a means for positioning the device adjacent the heat source. When the device is disposed adjacent the heat source, the heat generated by the heat source regenerates the desiccant. As used herein, the term xe2x80x9cadjacentxe2x80x9d means in close proximity sufficient for heat from the heat source to reach the device, preferably oriented in a region of the electronic enclosure generally above the heat source so that heated air from the bulb comes in contact with device.
In a preferred embodiment, the device of the present invention may incorporate one or more components which help to heat the air as it passes through the device. For example, the air-impermeable, water vapor permeable layer may be dark in color to faciliate absoprtion of energy generated by the bulb. Further, optional metal or other thermally conductive components may be incorporated in or with the device to absorb heat and transfer such heat to the air within the device.
The device of the present invention acts to reduce moisture by the transport of air through the device, particularly during thermal cycling of the heat source. When the heat source is energized, the air in the enclosure expands and passes through the desiccant chamber in the moisture reducing device through the vent, whether or not the vent is covered with an air permeable layer. The heated air continues through the device into the diffusion tube or channel. As the temperature rises due to the energy generated by a heat source, the desiccant regenerates (i.e., dries) and releases water vapor into the air which is then expelled into the external atmosphere through the diffusion tube. When the heat source is de-energized (e.g., turned off), the air inside the enclosure cools and contracts, and ambient air is drawn into the enclosure through the device and is dried by the regenerated desiccant. To prevent the entrance of liquids such as water and the like, an air-permeable, liquid water impermeable layer, preferably being a hydrophobic layer or more preferably an oleophobic layer, is placed at the opening of the enclosure where the device is positioned to contact the external atmosphere. A second, air-permeable layer inside the enclosure covering the internal vent opening may optionally be incorporated as well to assist in regulating the flow of air in the device.